I am not a vet and can't tell for sure. It looks like a skin abrasion. But it also could be something broken or sprained. Keep a close watch on how the frog moves and watch for infection at the least. (It's pretty red, it might already be infected. You can try putting a drop of NON PAINKILLING neosporin or bactine on the knee. Vet would be a good idea to be safe.